1. Field of the Invention
The present invention relates to structures adapted to attenuate sound normally accompanying the flow of a fluid medium such as air or gas streams in a confined place, and more particularly to an acoustical gas flow silencer which may be typically used in heating, ventilating and air conditioning systems, power plants, engine intakes and exhausts, process blowers and compressors, etc. The silencer of the present inventors may include sound absorptive (dissipative) elements as well as reactive (nondissipative) configurations and may function as a combined dissipative and reactive attenuator or as a purely reactive one.
2. Description of the Prior Art
One type of silencer that is in common use is a splitter silencer which generally consists of baffles containing sound absorptive materials, of varying lengths and thicknesses, disposed parallel to the direction of the fluid stream flow. The sound absorptive materials are usually protected by perforated metal sheets or screens and, for very high velocity applications, by additional materials. Because of this silencer's reliance on sound absorptive material for the attenuation of noise, it may generically be called a "dissipative" type silencer, and the term "dissipative" will be used herein to define such a silencer. One advantage of the "dissipative" type of splitter silencer is that with good aerodynamic design it has a relatively low pressure drop. This feature makes the silencer particularly well-suited for applications involving large volumes of fluid flow. It will be understood that pressure drop is always an important factor in that the lesser the amount of energy required to push fluids through the silencers and other system components, the more energy is available for other purposes such as generating electricity or producing other marketable goods. Pressure drop is, of course, also very important in situations where air supply is marginal to begin with. Another advantage of dissipative type splitter silencers is that they are generally effective in attenuating noise in a wide range of both high and low frequencies.
There is, however, a significant shortcoming associated with the use of dissipative type splitter silencers in "dirty" environments such as steel plants, food processing operations, coal mines, aluminum reduction plants, power plants, and in particular, coal fired power plants which generate a significant amount of fly ash. More particularly, when dissipative type splitter silencers are used in such environments, the perforations of the silencer's absorptive surfaces tend to become clogged by the particulate contaminants contained in the fluid passing through the silencer. This clogging results in significant losses in the noise reduction characteristics of the silencer, and so, undesirably frequent cleaning of the sound absorptive surfaces is required.
With certain fan or air/gas handling applications, as in power plants, for instance, the undesirable noise encountered is often due to certain discrete tones which may be attributable to the fan blade pass frequency. These frequencies usually lie in a relatively broad low frequency range such as from about 100 to 500 Hz. A particular type of dissipative splitter silencer which has been suggested for use in environments such as power plants is one in which the absorptive surfaces thereof are largely contained within pockets on each side of the splitter and are separated by means of a solid spline. However, this type of silencer, in addition to a clogging problem, also suffers from a further related problem. More particularly, the pockets of this type of splitter silencer, which are essentially at right angles to the direction of fluid flow, act as collectors for the particulate contaminants in the fluid. Thus, in addition to clogging the sound absorptive surfaces inside the pockets, they facilitate the build-up of accumulating contaminants. Further, the typical construction of these particular silencers is such that ready access can not be had to the portions of the silencer where the fluid contaminants have collected so as to hinder cleaning and declogging of the silencer. It will be noted that the noise reduction characteristics of the silencer will be seriously impaired by such accumulations. This is particularly significant in power plants. Power plants are constructed so as to be operative for periods on the order of forty or more years. Silencers, therefore, should be useful and accessible for maintenance for the same length of time. It will be appreciated that if a power plant had to shut down for any reason, the economic and social impact can be enormous.
From the above, it will be understood that a silencer which will attenuate the relatively broad low range of frequencies typically encountered in power plants, independent of the functioning of potentially clogged dissipative components, i.e., the perforated or screened plates covering the acoustically absorptive material, is highly desirable.
Another type of silencer in use is commonly called a "reactive" type silencer. As used herein a "reactive" type silencer refers to a silencer which is not dependent on the presence of sound absorptive or flow resistant materials. Reactive silencers attenuate sound predominantly by virtue of volumetric relationships and the reflection of energy, rather than by use of sound absorptive materials, thus avoiding the above-mentioned clogging problem associated with dissipative type silencers. However, reactive type silencers generally operate effectively only over a relatively narrow frequency range, and thus, do not provide an optimum solution to noise reduction in environments such as power plants.
Accordingly, it is an object of the present invention to provide a sound silencer which, while employing sound absorptive material, is so constructed so as to retard clogging, minimize the need for cleaning, and maintain effective noise attenuation over a wide range of high and low frequencies for longer periods of time than known silencers.
It is another object of the present invention to provide a sound silencer having the above characteristics which can effectively attenuate noises in a relatively wide range of frequencies even when the perforated plates or screens covering the acoustically absorptive material are clogged by contaminants such as fly ash in the fluid stream which flows through the silencer.
It is a further object of the present invention to provide a sound silencer having the above characteristics and which also maintains a low degree of pressure drop.
It is another object of the present invention to provide a sound silencer which is constructed so as to provide ready access to the portions thereof where contaminants may collect for easy cleaning thereof when necessary.
It is another object of the present invention to provide a sound silencer of the reactive type as defined above, i.e., which does not require any sound absorptive materials or surfaces, which effectively attenuates sound over a relatively broad frequency range while retaining low pressure drop characteristics.